Modern semiconductor device manufacture involves process steps that need to be performed with a high degree of control of the process conditions. In fact, failure to adequately control the process conditions can result in lower device yield and/or poor device performance. An example of a semiconductor wafer processing step in which a high degree of control of the process conditions is necessary is the transfer of fine patterns onto a workpiece such as a silicon wafer or other suitable substrate.
Photolithography is typically used to achieve the pattern transfer. Here, the workpiece such as a silicon wafer is covered with a light-sensitive material called photoresist. Light is projected onto the wafer through a mask and an optical system to selectively expose certain areas of the photoresist. The wafer is baked in a post exposure bake (PEB) step to activate but also diffuse the photosensitive compounds in the resist. This PEB step is critical for photolithography, and it enables very fine pattern transfer. Typically, the PEB step involves baking a wafer at 120° C. for 120 seconds. It is important to note that the actual temperature trajectory experienced by the wafer has a transient regime where the wafer temperature rises rapidly (usually in about 10-20 seconds) from ambient to about the temperature set point, and a near steady-state regime towards the end of the bake step (usually after about 30-60 seconds) where the temperatures are not changing significantly. The temperature regimes can be seen in FIG. 1 where there is shown a plot of temperature as a function of time at several locations on a semiconductor wafer subjected to a typical PEB process. It is also important to note that different locations on the wafer typically experience different temperature trajectories.
A central objective in semiconductor device manufacturing is to ensure reliable and repeatable pattern transfer. The common metric for this objective is the critical dimension (CD), which is the width of specific features printed on the wafers. It is very important to ensure that the CD is as close to the specified width as possible, preferably at all points on the wafer. In other words, there is a need for tightly controlling the CD uniformity across the wafer and making the mean CD as close to the target CD as possible. It is well known that the entire temperature trajectory, including both the transient and the near steady-state components, in the PEB step has a great impact on CD. Thus, controlling the temperature trajectories across the wafer is a means of controlling across wafer CD.
Some modern PEB plate designs have several control parameters, α=[α1, α2, . . . αN], which directly influence the spatial behavior of the temperature profiles experienced by the wafer. Some PEB plates even have multiple heating elements or zones that can be individually controlled. These include but are not limited to proportional integral derivative (PID) gains for each zone, airflow settings, and set-point adjustments for each zone. Other important parameters that cannot be easily controlled include the proximity distance between the plate and the work piece, the leveling of the work piece, the speed and direction of the wafer handling robotics, etc.
The current standard practice for calibrating temperature profiles for PEB processes is limited to the far steady-state regimes, and the calibration typically uses a trial and error procedure. Far steady state refers to the time after the temperatures across the wafer exhibit substantially no observable variations; this is usually about 10 or more minutes for PEB processes. In other words, the “far steady state” conditions cannot be achieved during the 60-120 seconds of normal processing of production wafers. To measure the “far steady state,” a special wafer outfitted with temperature sensors is placed on the bake plate manually or by robotics. The temperature sensors are typically tethered to an outside instrument that can record their temperature as a function of time. An operator waits a certain amount of time for the temperature transients to decay and the temperatures of the wafer to reach steady state. The steady-state temperature data is analyzed. Typically, averages of these temperatures are determined in various zones or regions. These temperature averages are compared to the desired temperature value or set point. Based on calculated differences, a look-up table is used to adjust the control parameters such as PID gains or offsets, etc. on the bake-plate. In some cases, these parameters can be adjusted by trial and error, where small changes are introduced, and, after the system has time to settle, new readings are collected, and so on. This trial and error procedure can even be automated using typically heuristic algorithms.
In summary, the standard technology practice for PEB has two characteristics. First, the calibration is conducted in the far steady state regime of 10 or more minutes into the bake step. Second, actual production wafers never experience the far steady-state regime because the typical bake times are almost never longer than about 120 seconds. This is because some of the resist components diffuse during PEB, and long PEB times would severely distort the fine features one attempts to transfer during the lithography process.
There is a need for improved methods for controlling and/or calibrating transient and steady state temperature profiles for post exposure bake processes. There is also a need for improved methods for controlling and/or calibrating transient and steady state temperature profiles for other types of substrate processing applications and processes. Examples of applications are processing substrates for manufacturing electronic devices such as integrated circuits on semiconductor wafers, mask for lithography processes, and substrates for flat panel displays. Examples of other types of processes are processes such as plasma etch processes, chemical vapor deposition processes, rapid thermal anneal processes, and ion implantation processes. These processes suffer similar problems and consequently there is a need for improved methods for controlling or calibrating transient and steady state temperature profiles for workpieces used therein.